For decreasing the weight of a flat panel display typically represented by a liquid crystal display device, it has been studied so far to decrease the thickness of a substrate, and liquid crystal display devices at present are manufactured by using a glass substrate having a thickness of about 0.5 mm to 1.1 mm. However, use of a glass substrate at the thickness less than that described above results in a problem that it tends to be cracked during manufacture or during use. As one of countermeasures, development has been proceeded for a liquid crystal display device of using a plastic substrate instead of the glass substrate.
However, while the heat resistance of the glass substrate is about 600° C., the plastic substrate has the heat resistance usually of about 200° C. and involves a problem that the heat resistance is low. The temperature for forming a thin film transistor is about 300° C. for an amorphous silicon (a-Si) TFT and about 500° C. for a low temperature polysilicon (LTPS) TFT, which is a temperature much higher than the heat resistance of the plastic substrate. Accordingly, as one of countermeasures, a method of lowering the TFT forming temperature has now been under study.
Further since the plastic substrate is soft and flexible different from the glass substrate, it involves a drawback that a line provided for current glass substrates cannot be used as it is. As a countermeasure, it is considered to change the line from a line corresponding to the glass substrate to a roll-to-roll system. As a method of using the current line as it is, a method of transferring TFT formed on a glass substrate to a plastic substrate has been under study. The method of forming and transferring TFT on the glass substrate includes a method of decreasing the thickness by etching the glass substrate or a method of previously forming a separation layer to a glass substrate and conducting separation after formation of TFT. Each of them is a method of reducing the thickness of a glass portion formed with TFT into a thin film and re-locating the same on a plastic substrate. Another transfer method includes a method of bonding a plastic substrate on a glass substrate, forming TFT thereon and then separating the glass substrate.
While the method of once forming TFT on the glass substrate and then decreasing the thickness of the glass has an advantage capable of using the current line and using a current process temperature as it is, when an etching step is adopted for the step of reducing the thickness of the glass substrate, almost of the glass substrate is lost wastefully to result in a drawback of increasing the cost. Further, the method of bonding the plastic substrate on the glass substrate, forming TFT thereon and then conducting separation involves a problem in view of the heat resistance of the plastic substrate and, accordingly, results in a drawback that the process temperature has to be lowered and a device of excellent characteristics cannot be obtained.
The following patent documents are incorporated herein for the purpose of reference:
JP 2737330, JP-A Nos. 2000-243943, 2000-284243, 2000-284303, 2001-125082, 2001-290138, 2002-33464, 2002-31818, 2002-258252, 2006-237542, 2002-100790 (corresponding to US. 2002/0037605A), and 2001-290138